Olefins are primarily generated by heat degradation of naphtha at a high temperature, which is a by-product obtained from a petroleum refining process. Since olefins, which are industrially important and serve as the basis of the petrochemical industry, are commonly produced together with paraffins such as ethane and propane, the techniques for separating the two are very important in the relevant industries.
A typical distillation method has been widely employed for separating a mixture of olefin/paraffin, such as ethylene/ethane and propylene/propane. However, since the molecular size and physical properties (e.g., relative volatility) of olefin are similar to those of paraffin, the separation of the two requires tremendous resources in terms of equipment and energy.
For example, the distillation method currently used in the art needs to operate a distillation column, which has about 120 to 160 stages, at a low temperature of −30° C. and a high pressure of about 20 Pa for the separation of ethylene/ethane. Further, such distillation method runs a distillation column, which has about 180 to 200 stages, at a low temperature of −30° C. and a several pressure with a reflux ratio of over 10 for the separation of propylene/propane. Therefore, there is a need to develop a new method of separating alkene/alkane hydrocarbons that can substitute the conventional distillation method.
As an alternative method for the conventional distillation method, a separation method using a membrane has been suggested. The membrane technique has made significant progresses during past several decades in separating gaseous mixtures such as nitrogen/oxygen, nitrogen/carbon dioxide and nitrogen/methane.
However, the typical gaseous separation membrane does not usually succeed in separating the mixture of olefin/paraffin hydrocarbons due to their similar molecular and physical properties. As a membrane showing high separation efficiency for the olefin/paraffin mixture, a facilitated transport membrane, which is based on a different concept from the typical gaseous membrane, has been developed.
The separation of a mixture using a membrane is accomplished by using the difference in permeability of each ingredient consisting the mixture. Most membrane materials show an inverse correlation between permeability and selectivity, which results in limiting their application. However, if a facilitated transport phenomenon is applied to the membrane technique, it will be possible to simultaneously increase the permeability and selectivity, thus enlarging its range of application. When the membrane contains a carrier capable of reacting selectively and reversibly with a specific ingredient in the mixture, an additional mass transport occurs due to a reversible reaction between the carrier and the specific ingredient. This results in increasing the efficiency of the whole mass transport. Therefore, the whole mass transport can be described as a sum of mass transport according to the Fick's law, which is caused by the reversible reaction of the carrier. This phenomenon is referred to as facilitated transport.
A supported liquid membrane, which is one of the membranes formed by employing the concept of facilitated transport, has been developed. The supported liquid membrane is prepared by dissolving a carrier capable of promoting the mass transfer in a solvent (e.g., water) and filling a porous membrane with the resulting solution. Such supported liquid membrane is generally successful.
For example, the Steigelmann and Hughes references (U.S. Pat. Nos. 3,758,603 and 3,758,605, respectively) disclose a supported liquid membrane, wherein the selectivity for ethylene/ethane ranges from 400 to 700 and the permeability to ethylene is 60 GPU (1 GPU=1×10−6 cm2 (STP)/cm2·sec·cmHg). Such permeable separation efficiencies are considered to be satisfactory. However, since such supported liquid membrane applies the facilitated transport efficiency only under a wet condition, it is not possible to maintain high permeable separation efficiency during a long-term operation due to the eventual loss of the solvent and the lowering of the separation efficiency.
In order to solve the above problem associated with the supported liquid membrane, U.S. Pat. No. 4,318,714 issued to Kimura, et al. discloses an ion-exchange membrane exhibiting facilitated transport of a certain gas by attaching a suitable counter-ion to the ion-exchange membrane matrix, which reacts reversibly with the gas to be separated. However, such ion-exchange membrane exhibits the facilitated transport property only under a wet condition similar to the supported liquid membranes described in the Steigelmann and Hughes references.
U.S. Pat. Nos. 5,015,268 and 5,062,866 disclose polymeric membranes for separating aliphatically unsaturated hydrocarbons from hydrocarbon mixtures. The polymeric membrane comprises a hydrophilic polymer such as a polyvinylalcohol, which contains metals capable of being in complex with the aliphatically unsaturated hydrocarbons. However, such polymeric membranes can exhibit satisfactory separation efficiency only when a feed stream is saturated with steam by bubbling it with water before contacting the membrane or swelling the membrane by using ethylene glycol or water.
All exemplary membranes described above must be maintained under a humidified condition so that they contain water or a similar solvent. In case of utilizing these membranes for the separation of a dry hydrocarbon gaseous mixture (e.g., olefin/paraffin) having no solvent such as water, solvent loss is inevitable. Thus, there has been developed a method for supplementing a solvent periodically in order to keep the membrane at a constant wet condition. However, it is impossible to apply such method in a practical manner and the prepared membranes are typically unstable.
U.S. Pat. No. 4,614,524 issued to Kraus, et al. describes a water-free immobilized liquid membrane for facilitated transport of aliphatically unsaturated hydrocarbons. The membranes are prepared by chemically bonding transition metal ions to a semi-permeable ion exchange membrane (e.g., Nafion) and are then plasticized by treatment with glycerol. However, their selectivity for a dry ethylene/ethane mixture is too low (i.e., about 10 under ambient condition) for practical use. Further, the plasticizer becomes lost with time and the membrane shows no selectivity when it does not undergo plasticization.
As described above, the conventional polymeric membranes cannot separate the mixture of olefin/paraffin hydrocarbons having similar molecular and physical properties. Thus, there has been a need to develop a facilitated transport membrane capable of selectively separating olefin from the mixture. However, the conventional transport membranes have to maintain the activity of their carriers by employing several methods such as filling a porous membrane with a carrier-containing solution, adding a volatile plasticizer, saturating a feed gas with steam and the like. Further, these facilitated transport membranes are not practical since their constituents become lost over time, thus deteriorating the stability of the membrane. In addition, the liquid solvent, which is provided periodically to the membrane in order to maintain its activity, has to be removed from the final product.
Recently, there have been also developed solid polymer electrolyte membranes containing a carrier complex in the form of an ionic metal salt, which is dissolved not in water or other solvent but in a solid polymer (see U.S. Pat. No. 5,670,051 issued to Pinnau, et al.; and Korean Patent No. 315896 issued to the present inventors). The solid polymer electrolyte membranes can be prepared, e.g., by coating a solid polymer electrolyte consisting of a transition metal salt such as AgBF4 and a polymer such as a poly(2-ethyl-2-oxazole) (POZ) having an amide group allowing the metal ion to be coordinated, on a porous support. This solid polymer electrolyte membrane (e.g., an AgBF4/POZ membrane) shows a high initial selectivity of 50 or more for the separation of olefin from an olefin/paraffin mixture. However, there is a problem in that the silver ion adopted as a carrier in the membrane has a tendency of being reduced to Ag0-particles undergoing agglomeration by themselves, thereby continuously decreasing the selectivity of the membrane over time. Further, the silver ion is chemically unstable to easily react with a sulfur-containing compound, acetylene or a hydrogen gas, etc., which generally exists in a dry hydrocarbon gas mixture, to lose its activity as a facilitated transport carrier for olefins.
Therefore, there is a need to develop a membrane for olefin/paraffin separation, which comprises chemically stable olefin carrier, thereby being able to maintain its activity over a long operational time under dry feed gaseous conditions.